Add constants.inc and start weaning off __DIR__

[stoyan]

the result of dumping the constants:

    [APP_ROOT] => /var/www/webpagetest
    [WWW_PATH] => /var/www/webpagetest/www
    [ASSETS_PATH] => /var/www/webpagetest/www/assets
    [VER_WEBPAGETEST] => 21.07
    [VER_TYPOGRAPHY_CSS] => d5698bbc95c8ee894bacb869a2f2c8db
    [VER_LAYOUT_CSS] => f25358772e3c9df79b64b9e42c9a6e2a
    [VER_CSS] => 78d3606c1075a66147605bffcee74af1
    [VER_HEADER_CSS] => 697fe9b78a9b4d678f1b1f5e10003158
    [VER_ACCOUNT_CSS] => f5573738ecc2f0501fdf5d499652275d
    [VER_PRICE_CHANGER_JS] => 554bd234290355d73f3dfeba7afb106e
    [VER_FAQ_JS] => 806baf80caa10bf70b8728b24646d8f2
    [VER_BUTTON_CSS] => b891260d9a5598a3f00b133a976691d1
    [VER_JS] => 9d569a69e7d08a09fa85457f03309e1f
    [VER_JS_TEST] => ebb31ca6dd55baee8528e2430bcebd46
    [VER_JS_RUNNING] => 816b08ba201d238d171a00f583fc0efa
    [VER_JS_ACCOUNT] => 6ce5743d45b52529092c48b81ed7547a
    [VER_JS_ESTIMATE_TAXES] => d87beb152cc2c8c266b9e29dfa2f5f68
    [VER_JS_COUNTRY_LIST] => 26dbe29586f62719390d604a96a9b12a
    [UNKNOWN_TIME] => -1

[jefflembeck]

This will be a very nice change. We're going to want too make sure we use constant('NAME') as well, because of upcoming PHP version changes. Using CONSTANTS without it is deprecated.